ASSESSMENT OF EXOTIC AND INVASIVE PLANT SPECIES IN SRI LANKA’s FLORA AND
Carla C Bossard
St. Mary’s University of California, USA
Globally, exotic naturalized plants that behave invasively and occupy
wildlands are responsible for greater losses of biodiversity than any
other factor except habitat loss and direct exploitation of plant species
by people. About 1 - 2% of naturalized exotic species become invasive
in behavior, infesting and sometimes destroying parks, preserves and refuges.
This occurs because the invasive plant species: have no natural enemies
or diseases present; spread rapidly due to high reproductive and dispersal
capabilities; and out compete native species.
Publications relating to the Sri Lankan flora, and Master’s theses available
in the library of Dept. of Forestry and Enviromental Science at the University
of Sri Jayewardenepura regarding various Sri Lankan biological communities,
provide the opportunity for an examination of Sri Lanka’s exotic flora
and an initial attempt to assess their impact on Sri Lanka’s plant communities.
Tallying herbs and woody species by life form, habitat preference and
origin (indigenous, endemic or exotic) revealed the following. Twenty-five
percent of Sri Lanka’s flora is exotic species (15.6% herbacious and 9.4%
woody species) not including cultivated species not known to escape. Herbacious
(34.6%) and liana’s / vines (19.5%) are the life forms with the highest
percentage exotics. The largest proportion of the total number of exotic
species is found in disturbed (22.6%) and wet forest (18.4%) habitats.
Amongst herbacious species the proportion of exotics is also high in aquatic
habitats (44.2%). Certain plant taxa contain species which readily naturalize
in Sri Lanka. Four plant families, Fabaceae, Verbeneaceae, Myrtaceae,
and Rutaceae contain >55% of all woody exotics. The Poaceae, alone account
for 28.6% of all exotic herbs.
Species lists from Masters theses done on two wet zone forest reserves
indicate seed banks contain about 30% exotic species, while mature wet
zone forests contain only 5 - 11% exotic species. If seed banks contain
the seeds of exotic species underwhich the native or mature forest species
can not germinate or survive, this would be cause for much concern. It
is certainly something that merits further research.
Initial observations in the field were made of % cover by indigenous +
endemic and exotic species along 7 randomly located transects in several
different habitat types. Field measurements indicate Sri Lankan wetzone
forests are highly resistant to exotics and inhibit potential invasive
behavior. Except in young forest gaps (where up to 20% of cover can be
exotic species) almost all cover is indigenous+endemic species. In disturbed
lands, wet and low zone grasslands and aquatic habitat (fresh water pond)
in Sri Lanka the case was quite different. Greater than 80% of the cover
was exotic species in random transects sampled in these three habitat
The total impact of the diverse assemblage of exotic species in Sri Lankan’s
should be of some concern. The exotics are competing with each other in
some habitats which prevents any one of them from taking over and forming
a monospecific stand, but the indigenous species are forced out regardless.
How widespread this is in other habitats in Sri Lanka merits further research
so management recommendations can made.
Department of Forestry and Environmental Science,
of Sri Jayewardenepura,Sri Lanka. 1999. All rights reserved.